Run more steps in CI

[ondrej-fabry]

To improve the quality of the code that is merged to master, the CI pipeline should perform more tests or checks for common things developers forget to update.

• verify contents of go.mod file are up-to-date and go.sum is not missing any entry
  • Update ci.yaml #61
• add golangci-lint for linting and code checks
  • Run golangci-lint in CI #69
• add base for integration/e2e tests against real VPP instances
  • Introduce VPP integration tests #75
• run examples against master version of VPP and 2-3 recent VPP releases

[dwallacelf]

Do we need a docs verification task?

I was able to review PR#70 by going to the 'Files Changed' tab and selecting '...'/'View file' for docs/troubleshooting.md which showed that the markdown rendered correctly.

However, if the syntax was incorrect, does anything in the CI run *.md files through the github renderer and mark it as not mergeable?

As an optimization, it would be good to skip the build verification tasks for docs only PR's, but may not be worth implementing if it make the CI too complex.

[ondrej-fabry]

All tasks complete as part of release v0.7.0. For the rest separate issues were created for tracking them.